Redwood Neuroscience
Title: “Callosal
Projections to Retinotopic Visual Cortex Measured
with DTI and fMRI”
Bob
Dougherty
Department
of Psychology
Abstract:
Post mortem studies of human callosal
connections in occipital lobe patients show a selective pattern of
connectivity: callosal fibers mainly project to the
vertical meridia and foveal
confluence of early visual ares (Clarke, 1990). We
used diffusion tensor imaging (DTI) to estimate callosal
connections between right and left occipital lobes. These results were
integrated with detailed retinotopic maps (fMRI) in order to evaluate the sensitivity and specificity
of DTI path tracing.
Methods: Retinotopic regions
of visual cortex were mapped using rotating wedge and expanding ring stimuli,
which create traveling waves of neural activity measured with BOLD fMRI. The cortical sheet of gray matter was segmented from
high-resolution T1 MR Images. DTI was measured using a diffusion-weighted EPI
pulse sequence with ~2x2x2mm voxels. Path tracing
along the principal diffusion direction (Mori 2002) was used to identify the callosal projections to retinotopic
visual cortex. The endpoints of the DTI paths, representing clusters of neural
fibers that terminate in the gray matter, were extrapolated to the cortical
sheet for visualization along with the fMRI data.
Results: We observed callosal
projections primarily along the vertical meridia of
early visual areas. The vertical meridian shared by V3d and V3A was especially
consistent across subjects. We also observed pojections
to area
Conclusions: The results agree with post mortem data,
but depict a fairly sparse pattern of connectivity in the visual cortex. This
may reflect resolution limits of our DTI measures. However, despite the lack of
sensitivity, current DTI techniques do appear to be quite specific, giving few
false-positives.